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Abstract:

A hybrid vehicle includes an engine and a motor each for generating
vehicle driving power. An operation region indicated by rotational speed
and torque of the engine includes: an normal region in which a fuel
injection amount is calculated in accordance with a stoichiometric
air-fuel ratio; and an amount increase region in which an amount of fuel
is increased compared with the normal position so as to suppress
temperature increase of a catalyst. A control device calculates a total
required power of the vehicle depending on the vehicle state, and
determines an engine operation point in accordance with the total
required power. When the engine operation point is in the OT amount
increase region, the engine operation point is changed to fall within the
normal region by decreasing the engine output power. An output power of
the motor is determined to compensate the decrease of the engine output
power and secure the total required power.

Claims:

1. A control device for a hybrid vehicle including an internal combustion
engine and a motor each for generating vehicle driving power, wherein an
operation region indicated by rotational speed and torque of said
internal combustion engine includes a first operation region in which a
fuel injection amount is calculated in accordance with a stoichiometric
air-fuel ratio, and a second operation region in which an amount of fuel
is increased to exceed the fuel injection amount that is in accordance
with said stoichiometric air-fuel ratio so as to suppress temperature
increase of a catalyst provided in an exhausting system of said internal
combustion engine, and said control device calculates a total required
power of said hybrid vehicle in accordance with a vehicle state,
determines an output power of said internal combustion engine such that
the torque and the rotational speed of said internal combustion engine
are included in said first operation region in all vehicle states, and
determines an output power of said motor based on the output power of
said internal combustion engine so as to secure said total required
power.

2. The control device for the hybrid vehicle according to claim 1,
wherein said hybrid vehicle further includes a starter motor for motoring
said internal combustion engine when starting said internal combustion
engine, and when starting said internal combustion engine, said control
device sets the fuel injection amount of said internal combustion engine
in accordance with said stoichiometric air-fuel ratio, and controls said
starter motor to generate positive torque for said motoring during a
period of time until the rotational speed of said internal combustion
engine reaches a target rotational speed upon the starting.

3. The control device for the hybrid vehicle according to claim 1,
wherein said hybrid vehicle further includes a starter motor for motoring
said internal combustion engine when starting said internal combustion
engine, and when starting said internal combustion engine, said control
device sets the fuel injection amount of said internal combustion engine
in accordance with said stoichiometric air-fuel ratio, and controls said
starter motor to settle a rotational speed of said starter motor at a
steady-state rotational speed after the rotational speed of said starter
motor is temporarily increased to exceed the steady-state rotational
speed, said steady-state rotational speed being a rotational speed when
the rotational speed of said internal combustion engine reaches a target
rotational speed upon the starting.

4. The control device for the hybrid vehicle according to claim 1,
wherein said hybrid vehicle further includes: a power storage device for
storing electric power used to drive said motor; and a power generating
structure for generating electric power for charging said power storage
device, using an output of said internal combustion engine, and when
stored energy of said power storage device is lower than a reference
value, said control device changes an operation point of said internal
combustion engine so as to increase the rotational speed of said internal
combustion engine with the output power of said internal combustion
engine being constant.

5. The control device for the hybrid vehicle according to claim 1,
wherein said hybrid vehicle further includes: a power storage device for
storing electric power used to drive said motor; and a power generating
structure for generating electric power for charging said power storage
device, during vehicle traveling, and said control device determines
based on the traveling state whether or not it is necessary to perform
charge level increasing control for said power storage device so as to
prepare for a high output request to said internal combustion engine, and
controls said power generating structure to increase stored energy of
said power storage device when it is determined necessary to perform said
charge level increasing control.

6. The control device for the hybrid vehicle according to claim 1,
wherein said control device determines a ratio of output powers of said
internal combustion engine and said motor in said total required power,
and when an operation point of said internal combustion engine in
accordance with the ratio of powers determined is included in said second
operation region, said control device decreases the output power of said
internal combustion engine so as to change the operation point of said
internal combustion engine to fall within said first operation region,
and modifies the ratio of powers so as to increase the output power of
said motor in a reflection of the decrease of the output power of said
internal combustion engine for the change of said operation point.

7. A control method for a hybrid vehicle including an internal combustion
engine and a motor each for generating vehicle driving power, an
operation region indicated by rotational speed and torque of said
internal combustion engine including a first operation region in which a
fuel injection amount is calculated in accordance with a stoichiometric
air-fuel ratio, and a second operation region in which an amount of fuel
is increased to exceed the fuel injection amount that is in accordance
with said stoichiometric air-fuel ratio so as to suppress temperature
increase of a catalyst provided in an exhausting system of said internal
combustion engine, the control method comprising the steps of:
calculating a total required power of said hybrid vehicle in accordance
with a vehicle state; determining an output power of said internal
combustion engine such that the torque and the rotational speed of said
internal combustion engine are included in said first operation region in
all vehicle states; and determining an output power of said motor based
on the output power of said internal combustion engine so as to secure
said total required power.

8. The control method for the hybrid vehicle according to claim 7,
wherein said hybrid vehicle further includes a starter motor for motoring
said internal combustion engine when starting said internal combustion
engine, and when starting said internal combustion engine, the fuel
injection amount of said internal combustion engine is set in accordance
with said stoichiometric air-fuel ratio, and said starter motor is
controlled to generate positive torque for said motoring during a period
of time until the rotational speed of said internal combustion engine
reaches a target rotational speed upon the starting.

9. The control method for the hybrid vehicle according to claim 7,
wherein said hybrid vehicle further includes a starter motor for motoring
said internal combustion engine when starting said internal combustion
engine, and when starting said internal combustion engine, the fuel
injection amount of said internal combustion engine is set in accordance
with said stoichiometric air-fuel ratio, and said starter motor is
controlled to settle a rotational speed of said starter motor at a
steady-state rotational speed after the rotational speed of said starter
motor is temporarily increased to exceed the steady-state rotational
speed, said steady-state rotational speed being a rotational speed when
the rotational speed of said internal combustion engine reaches a target
rotational speed upon the starting.

10. The control method for the hybrid vehicle according to claim 7,
wherein said hybrid vehicle further includes: a power storage device for
storing electric power used to drive said motor; and a power generating
structure for generating electric power for charging said power storage
device, using an output of said internal combustion engine, the control
method further comprising the step of changing an operation point of said
internal combustion engine when stored energy of said power storage
device is lower than a reference value, so as to increase the rotational
speed of said internal combustion engine with the output power of said
internal combustion engine being constant.

11. The control method for the hybrid vehicle according to claim 7,
wherein said hybrid vehicle further includes: a power storage device for
storing electric power used to drive said motor; and a power generating
structure for generating electric power for charging said power storage
device, during vehicle traveling, the control method further comprising
the steps of: determining based on the traveling state whether or not it
is necessary to perform charge level increasing control for said power
storage device so as to prepare for a high output request to said
internal combustion engine; and controlling said power generating
structure to increase stored energy of said power storage device when it
is determined necessary to perform said charge level increasing control.

12. The control method for the hybrid vehicle according to claim 7,
further comprising the step of determining a ratio of output powers of
said internal combustion engine and said motor in said total required
power, wherein when an operation point of said internal combustion engine
in accordance with the ratio of powers determined is included in said
second operation region, the step of determining the output power of said
internal combustion engine decreases the output power of said internal
combustion engine so as to change the operation point of said internal
combustion engine to fall within said first operation region, and the
step of determining the ratio of output powers modifies the ratio of
powers so as to increase the output power of said motor in a reflection
of the decrease of the output power of said internal combustion engine
for the change of said operation point.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a control device and a control
method for a hybrid vehicle, more particularly, suppression of exhaust
emission from an internal combustion engine of the hybrid vehicle.

BACKGROUND ART

[0002] A hybrid vehicle, which includes an engine and a motor as a driving
power source, can achieve zero emission traveling by traveling only using
the motor with the engine being stopped. Meanwhile, generally, at least
during high-speed traveling or acceleration in which driving power
becomes large, the hybrid vehicle travels with the engine being operated.
Hence, in order to remove emission from the exhaust gas of the engine, an
exhaust emission purifier such as a three-way catalyst is provided. In
the description below, such exhaust emission purifiers are also
collectively and simply referred to as "catalyst".

[0003] Japanese Patent Laying-Open No. 2007-237794 (PTL 1) describes a
technique for decreasing temperature of a catalyst when the temperature
of the catalyst becomes excessively high. Specifically, it is described
that when the level of state of charge (SOC) in the battery is high, the
motor is controlled to increase motor output so as to rotate the engine
at a predetermined rotational speed or more using the motor output, and
that when the level of the SOC is low, the air-fuel ratio of the engine
is enriched and engine output is increased to get out of a low-load
region.

[0004] Further, Japanese Patent Laying-Open No. 2004-204707 (PTL 2)
describes engine control for avoiding occurrence of knocking in a hybrid
vehicle. Specifically, when operating state of the engine is transitioned
from an initial state to a final state in response to a request for
high-load output, the engine is temporarily controlled to operate in a
transient state, which is an intermediate state between the initial state
and the final state. While operating in the transient state, the motor
generator operates to compensate insufficiency of the engine output.

CITATION LIST

Patent Literature

[0005] PTL 1: Japanese Patent Laying-Open No. 2007-237794

[0006] PTL 2:
Japanese Patent Laying-Open No. 2004-204707

SUMMARY OF INVENTION

Technical Problem

[0007] By enriching the air-fuel ratio of the engine as described in PTL
1, the temperature of the exhaust gas of the engine can be decreased,
thereby suppressing temperature increase of the catalyst. Hence, in order
to suppress temperature increase of the catalyst in the high output
region of the engine, it is known to increase an amount of fuel
(hereinafter, also referred to as "OT amount increase") so as to render
the air-fuel ratio richer than the stoichiometric air-fuel ratio.

[0008] However, when the increase in amount of fuel causes decrease of the
air-fuel ratio to fall below the stoichiometric air-fuel ratio, unburned
CO is generated. Accordingly, even when the catalyst is activated,
exhaust emission may be increased. In particular, in recent years,
regulations on emission are becoming severer. In order to satisfy
requirements of such regulations on emission, the increase in amount of
fuel may not be permitted to be applied.

[0009] The present invention has been made to solve such a problem, and
has its object to suppress exhaust emission of a hybrid vehicle without
causing insufficiency of vehicle driving power and exhaust emission
purifier's (catalyst's) excessively high temperature resulting from
increase of temperature of exhaust gas.

Solution to Problem

[0010] In a certain aspect of the present invention, in a control device
for a hybrid vehicle including an internal combustion engine and a motor
each for generating vehicle driving power, an operation region indicated
by rotational speed and torque of the internal combustion engine includes
a first operation region in which a, fuel injection amount is calculated
in accordance with a stoichiometric air-fuel ratio, and a second
operation region in which an amount of fuel is increased to exceed the
fuel injection amount that is in accordance with the stoichiometric
air-fuel ratio so as to suppress temperature increase of a catalyst
provided in an exhausting system of the internal combustion engine. The
control device calculates a total required power of the hybrid vehicle in
accordance with a vehicle state, determines an output power of the
internal combustion engine such that the torque and the rotational speed
of the internal combustion engine are included in the first operation
region in all vehicle states, and determines an output power of the motor
based on the output power of the internal combustion engine so as to
secure the total required power.

[0011] Preferably, the hybrid vehicle further includes a starter motor for
motoring the internal combustion engine when starting the internal
combustion engine. When starting the internal combustion engine, the
control device sets the fuel injection amount of the internal combustion
engine in accordance with the stoichiometric air-fuel ratio, and controls
the starter motor to generate positive torque for the motoring during a
period of time until the rotational speed of the internal combustion
engine reaches a target rotational speed upon the starting.
Alternatively, when starting the internal combustion engine, the control
device sets the fuel injection amount of the internal combustion engine
in accordance with the stoichiometric air-fuel ratio, and controls the
starter motor to settle a rotational speed of the starter motor at a
steady-state rotational speed after the rotational speed of the starter
motor is temporarily increased to exceed the steady-state rotational
speed, the steady-state rotational speed being a rotational speed when
the rotational speed of the internal combustion engine reaches a target
rotational speed upon the starting.

[0012] Preferably, the hybrid vehicle further includes: a power storage
device for storing electric power used to drive the motor; and a power
generating structure for generating electric power for charging the power
storage device, using an output of the internal combustion engine. When
stored energy of the power storage device is lower than a reference
value, the control device changes an operation point of the internal
combustion engine so as to increase the rotational speed of the internal
combustion engine with the output power of the internal combustion engine
being constant.

[0013] Alternatively, the hybrid vehicle preferably further includes: a
power storage device for storing electric power used to drive the motor;
and a power generating structure for generating electric power for
charging the power storage device, during vehicle traveling. The control
device determines based on the traveling state whether or not it is
necessary to perform charge level increasing control for the power
storage device so as to prepare for a high output request to the internal
combustion engine, and controls the power generating structure to
increase stored energy of the power storage device when it is determined
necessary to perform the charge level increasing control.

[0014] More preferably, the control device determines a ratio of output
powers of the internal combustion engine and the motor in the total
required power, and when an operation point of the internal combustion
engine in accordance with the ratio of powers determined is included in
the second operation region, the control device decreases the output
power of the internal combustion engine so as to change the operation
point of the internal combustion engine to fall within the first
operation region, and modifies the ratio of powers so as to increase the
output power of the motor in a reflection of the decrease of the output
power of the internal combustion engine for the change of the operation
point.

[0015] In another aspect of the present invention, in a control method for
a hybrid vehicle including an internal combustion engine and a motor each
for generating vehicle driving power, an operation region indicated by
rotational speed and torque of the internal combustion engine includes a
first operation region in which a fuel injection amount is calculated in
accordance with a stoichiometric air-fuel ratio, and a second operation
region in which an amount of fuel is increased to exceed the fuel
injection amount that is in accordance with the stoichiometric air-fuel
ratio so as to suppress temperature increase of a catalyst provided in an
exhausting system of the internal combustion engine. The control method
includes the steps of: calculating a total required power of the hybrid
vehicle in accordance with a vehicle state; determining an output power
of the internal combustion engine such that the torque and the rotational
speed of the internal combustion engine are included in the first
operation region in all vehicle states; and determining an output power
of the motor based on the output power of the internal combustion engine
so as to secure the total required power.

[0016] Preferably, the hybrid vehicle further includes a starter motor for
motoring the internal combustion engine when starting the internal
combustion engine. When starting the internal combustion engine, the fuel
injection amount of the internal combustion engine is set in accordance
with the stoichiometric air-fuel ratio, and the starter motor is
controlled to generate positive torque for the motoring during a period
of time until the rotational speed of the internal combustion engine
reaches a target rotational speed upon the starting. Alternatively, when
starting the internal combustion engine, the fuel injection amount of the
internal combustion engine is set in accordance with the stoichiometric
air-fuel ratio, and the starter motor is controlled to settle a
rotational speed of the starter motor at a steady-state rotational speed
after the rotational speed of the starter motor is temporarily increased
to exceed the steady-state rotational speed, the steady-state rotational
speed being a rotational speed when the rotational speed of the internal
combustion engine reaches a target rotational speed upon the starting.

[0017] Preferably, the hybrid vehicle further includes: a power storage
device for storing electric power used to drive the motor; and a power
generating structure for generating electric power for charging the power
storage device, during vehicle traveling. The control method further
includes the step of changing an operation point of the internal
combustion engine when stored energy of the power storage device is lower
than a reference value, so as to increase the rotational speed of the
internal combustion engine with the output power of the internal
combustion engine being constant.

[0018] Alternatively, the hybrid vehicle preferably further includes: a
power storage device for storing electric power used to drive the motor;
and a power generating structure for generating electric power for
charging the power storage device, during vehicle traveling. The control
method further includes the steps of: determining based on the traveling
state whether or not it is necessary to perform charge level increasing
control for the power storage device so as to prepare for a high output
request to the internal combustion engine; and controlling the power
generating structure to increase stored energy of the power storage
device when it is determined necessary to perform the charge level
increasing control.

[0019] More preferably, the control method further includes the step of
determining a ratio of output powers of the internal combustion engine
and the motor in the total required power. When an operation point of the
internal combustion engine in accordance with the ratio of powers
determined is included in the second operation region, the step of
determining the output power of the internal combustion engine decreases
the output power of the internal combustion engine so as to change the
operation point of the internal combustion engine to fall within the
first operation region. The step of determining the ratio of output
powers modifies the ratio of powers so as to increase the output power of
the motor in a reflection of the decrease of the output power of the
internal combustion engine for the change of the operation point.

Advantageous Effects of Invention

[0020] According to the present invention, the exhaust emission of the
hybrid vehicle can be suppressed without causing insufficiency of the
vehicle driving power and the exhaust emission purifier's (catalyst's)
excessively high temperature resulting from increase of the exhaust gas
temperature.

BRIEF DESCRIPTION OF DRAWINGS

[0021]FIG. 1 is a block diagram for illustrating a hybrid vehicle
including a control device according to a first embodiment of the present
invention.

[0022]FIG. 2 illustrates a configuration of an engine shown in FIG. 1, in
detail.

[0023]FIG. 3 is a nomographic chart showing a relation among rotational
speeds of the engine, a first MG, and a second MG of the hybrid vehicle.

[0024]FIG. 4 is a nomographic chart when the engine of the hybrid vehicle
is stopped.

[0025]FIG. 5 is a nomographic chart after starting the engine of the
hybrid vehicle.

[0026]FIG. 6 is a conceptual view illustrating operation regions of the
engine.

[0027]FIG. 7 is a flowchart illustrating the traveling control for the
hybrid vehicle in the first embodiment of the present invention.

[0028]FIG. 8 is a nomographic chart in an engine operation point before
changing in the first embodiment.

[0029]FIG. 9 is a nomographic chart in the engine operation point after
changing in the first embodiment.

[0030] FIG. 10 is a flowchart illustrating traveling control for a hybrid
vehicle in a second embodiment of the present invention.

[0031] FIG. 11 is a conceptual view illustrating change of an operation
point in the second embodiment.

[0032] FIG. 12 is a nomographic chart in the engine operation point before
changing in the second embodiment.

[0033] FIG. 13 is a nomographic chart in the engine operation point after
changing in the second embodiment.

[0034] FIG. 14 is a flowchart illustrating traveling control for a hybrid
vehicle in a third embodiment of the present invention.

[0035] FIG. 15 is a graph showing exemplary traveling control for the
hybrid vehicle in the third embodiment.

[0036] FIG. 16 is a waveform diagram illustrating normal engine starting
control for the hybrid vehicle.

[0037] FIG. 17 is a waveform diagram illustrating engine starting control
according to traveling control for a hybrid vehicle in a fourth
embodiment.

DESCRIPTION OF EMBODIMENTS

[0038] In the following, embodiments of the present invention will be
described in detail with reference to figures. It should be noted that
the same or corresponding portions are given the same reference
characters and are not described repeatedly in principle.

First Embodiment

[0039]FIG. 1 is a block diagram for illustrating a hybrid vehicle 100
including a control device according to a first embodiment of the present
invention.

[0040] Referring to FIG. 1, hybrid vehicle 100 includes a first MG (Motor
Generator) 110, a second MG 120, a power split device 130, a speed
reducer 140, a battery 150, a PM (Power train Manager)-ECU (Electronic
Control Unit) 170, an MG (Motor Generator)-ECU 172, and an engine 200
corresponding to an "internal combustion engine".

[0041] As will be apparent in the description below, PM-ECU 170 implements
traveling control, which is performed by a control device according to
the first embodiment of the present invention. It should be noted that
each of the ECUs such as PM-ECU 170 is configured to include a CPU
(Central Processing Unit) and a memory both not shown in figures, and is
configured to perform calculation process based on a detection value from
each sensor, by means of software processing in accordance with a map and
a program stored in the memory. Alternatively, at least a part of the ECU
may be configured to perform predetermined mathematical operation process
and/or logical operation process by means of hardware processing using a
dedicated electronic circuit or the like.

[0042] First MG 110, second MG 120, and engine 200 are coupled to one
another via power split device 130. Motive power generated by engine 200
is split into two paths by power split device 130. One of the paths is a
path for driving front wheels 160 via speed reducer 140. The other is a
path for driving first MG 110 for electric power generation. Hybrid
vehicle 100 travels using driving power from at least one of engine 200
and second MG 120.

[0044] Referring to FIG. 2, in engine 200, air taken in via an air cleaner
(not shown) flows through an intake pipe 210 and is then introduced into
a combustion chamber 202 of engine 200. Depending on an opening position
(throttle opening position) of a throttle valve 214, an amount of air to
be introduced into combustion chamber 202 is adjusted. The throttle
opening position is controlled by a throttle motor 212.

[0045] Fuel is stored in a fuel tank (not shown), is supplied via a fuel
pump (not shown), and is injected from an injector 204 to combustion
chamber 202. An air-fuel mixture of the air introduced from intake pipe
210 and the fuel injected from injector 204 is ignited using an ignition
coil 206, which is controlled in accordance with a control signal from an
ECU 400, and is burned.

[0046] Exhaust gas resulting from the burning of the air-fuel mixture is
exhausted to outer atmosphere through a catalyst 240 provided in an
exhausting system of engine 200. Catalyst 240 is representatively
provided in exhaust pipe 220. Catalyst 240 collectively represents
various types of exhaust emission purifiers.

[0047] Catalyst 240 is representatively formed of a three-way catalyst for
purifying emissions (harmful substances such as hydrocarbon (HC), carbon
monoxide (CO), and nitrogen oxide (NOx)) contained in the exhaust gas.
Catalyst 240, which carries a noble metal basically composed of alumina
and having platinum, palladium, and rhodium added thereto, is capable of
simultaneously providing oxidation reaction of hydrocarbon and carbon
monoxide and reduction reaction of nitrogen oxide. Catalyst 240 generally
has such a characteristic that it exhibits higher exhaust air
purification performance as the temperature becomes higher. However, when
the temperature thereof becomes excessively high, the characteristic may
be deteriorated and failure may take place. Hence, when catalyst 240 is
in a low temperature state in which catalyst 240 is inactive, catalyst
240 needs to be warmed up quickly by actively increasing the exhaust gas
temperature. On the other hand, after activation of catalyst 240, it is
necessary to control the exhaust gas temperature not to become too high
for the purpose of protection from overheat.

[0049] Engine coolant temperature sensor 208 detects temperature (engine
coolant temperature) TW of engine coolant. Airflow meter 216 is provided
in intake pipe 210 at an upstream relative to throttle valve 214, and
detects an intake air amount (amount of air taken into engine 200 per
unit time) Ga. Intake air temperature sensor 218 detects temperature
(intake air temperature) TA of the intake air. Air-fuel ratio sensor 222
detects a ratio of air and fuel in the exhaust gas. Oxygen sensor 224
detects oxygen concentration in the exhaust gas. Each of these sensors
sends a signal indicating its detection result to engine ECU 201.

[0050] Engine ECU 201 controls engine 200 in accordance with a control
target value from PM-ECU 170 of FIG. 1. Specifically, based on a signal
sent from each sensor as well as a map and a program stored in the ROM,
engine ECU 201 controls each of the elements of engine 200 such that each
of rotational speed and torque of engine 200 coincides with the control
target value. For example, engine ECU 201 controls ignition coil 206 to
ignite at an appropriate timing, and controls throttle motor 212 to
attain an appropriate throttle opening position. Further, engine ECU 201
controls injector 204 to inject an appropriate amount of fuel.
Specifically, based on the signals from air-fuel ratio sensor 222 and
oxygen sensor 224, the fuel injection amount is feedback-controlled to
attain an appropriate value of air-fuel ratio.

[0051] Referring to FIG. 1 again, first MG 110 is a three-phase
alternating current rotating electric machine having a U-phase coil, a
V-phase coil, and a W-phase coil. First MG 110 generates electric power
using the driving power of engine 200 that is split by power split device
130. The electric power generated by first MG 110 is used depending on
the traveling state of the vehicle and SOC (State Of Charge), which
indicates an amount of charges in battery 150.

[0052] For example, in the normal traveling, the electric power generated
by first MG 110 is directly used as electric power for driving second MG
120. On the other hand, when the SOC of battery 150 is lower than the
control target, electric power generated by first MG 110 is converted
from alternating current to direct current by an inverter described
later. Thereafter, the electric power is adjusted in voltage by a
converter described later and then is stored in battery 150. It should be
noted that the control target of the SOC may be a single SOC target value
or may be a certain range of SOC.

[0053] Second MG 120 is a three-phase alternating current rotating
electric machine having a U-phase coil, a V-phase coil, and a W-phase
coil. Second MG 120 is driven using at least one of the electric power
stored in battery 150 and the electric power generated by first MG 110.

[0054] Driving power of second MG 120 is transmitted to front wheels 160
through speed reducer 140. Accordingly, second MG 120 assists engine 200
or allows the vehicle to travel with the driving power from second MG
120. The rear wheels may be driven in place of or in addition to front
wheels 160. Namely, second MG 120 corresponds to a "motor" for generating
vehicle driving power.

[0055] At the time of regenerative braking of hybrid vehicle 100, second
MG 120 is driven by front wheels 160 through speed reducer 140 and
operates as an electric power generator. Thus, second MG 120 operates as
a regenerative brake for converting braking energy into electric power.
This electric power generated by second MG 120 is stored in battery 150.

[0056] First MG 110 and second MG 120 are controlled by means of, for
example, PWM (Pulse Width Modulation) control performed by an inverter
(not shown). It should be noted that a well-known, general technique can
be employed for a method of controlling first MG 110 and second MG 120
using the PWM control, and is therefore not repeatedly described more in
detail herein.

[0057] Power split device 130 is formed of a planetary gear including a
sun gear, pinion gears, a carrier, and a ring gear. The pinion gears are
engaged with the sun gear and the ring gear. The carrier supports the
pinion gears such that they are rotatable on their own axes. The sun gear
is coupled to the rotation shaft of first MG 110. The carrier is coupled
to the crankshaft of engine 200. The ring gear is coupled to a rotation
shaft of second MG 120 and speed reducer 140.

[0058] Engine 200, first MG 110, and second MG 120 are coupled to one
another via the planetary gear unit. Accordingly, the rotational speeds
of engine 200, first MG 110, and second MG 120 have a relation
represented by a straight line in a nomographic chart as shown in FIG. 3.

[0059] Thus, as shown in FIG. 4, when hybrid vehicle 100 travels using
only driving power of second MG 120 with engine 200 being stopped, the
rotational speed of the output shaft of second MG 120 becomes positive
and the rotational speed of the output shaft of first MG 110 becomes
negative.

[0060] When starting engine 200, as shown in FIG. 5, first MG 110 is
operated as a motor so as to crank engine 200 using first MG 110, whereby
the rotational speed of the output shaft of first MG 110 becomes
positive. Namely, first MG 110 can be operated as a "starter motor".

[0061] Referring to FIG. 1 again, battery 150 is generally constituted of
a battery pack configured such that a plurality of battery modules, each
formed by integrating a plurality of battery cells, are connected in
series. The voltage of battery 150 is, for example, about 200 V.

[0062] Battery 150 is charged with electric power generated by first MG
110 or second MG 120. Electric power stored in battery 150 can be used
for driving of first MG 110 and second MG 120. Namely, battery 150
corresponds to a "power storage device".

[0063] States of battery 150 such as a temperature state, a voltage state,
and a current state are detected by battery sensor 152. Battery sensor
152 collectively represents various types of sensors such as a
temperature sensor, a voltage sensor, and a current sensor. The electric
power charged to battery 150 is controlled not to exceed an upper limit
value WIN. Likewise, electric power discharged from battery 150 is
controlled not to exceed an upper limit value WOUT. Upper limit values
WIN, WOUT are determined based on parameters of battery 150 such as SOC,
temperature, and a rate of change in temperature.

[0064] As described above, engine 200 is controlled in accordance with the
control target value provided by PM-ECU 170. PM-ECU 170 and MG-ECU 172
are connected to each other such that they can communicate with each
other bidirectionally. PM-ECU 170 generates control target values (such
as torque command values) for first MG 110 and second MG 120 in
accordance with traveling control described below. In accordance with the
control target values sent from PM-ECU 170, MG-ECU 172 controls first MG
110 and second MG 120.

[0065] In hybrid vehicle 100, traveling control to attain traveling
suitable for a vehicle state is performed by PM-ECU 170. For example,
when starting to travel the vehicle and when traveling in a low speed,
hybrid vehicle 100 travels using output of second MG 120 with engine 200
being stopped as shown in the nomographic chart of FIG. 4. During a
steady-state traveling, as shown in the nomographic chart of FIG. 5,
engine 200 is started and hybrid vehicle 100 travels using the outputs of
engine 200 and second MG 120. In particular, by operating engine 200 at a
highly efficient operation point, hybrid vehicle 100 is improved in fuel
consumption.

[0066] As described above, in hybrid vehicle 100, first MG 110 can serve
as a "power generating structure" for generating electric power using the
output of engine 200. In order to obtain electric power to charge battery
150 and/or electric power to be consumed by second MG 120, the output of
engine 200 is increased as required and first MG 110 is controlled to
generate electric power using the output thus increased. For example,
when the SOC of battery 150 is decreased, engine 200 is started to charge
battery 150 even in an operating state (such as low-speed/low-load
traveling) in which engine 200 should have been stopped in the first
place.

[0067] Thus, in hybrid vehicle 100 according to the present embodiment,
the traveling control by PM-ECU 170 determines whether to operate engine
200 and determines the rotational speed and torque of engine 200 during
the operation.

[0068] The following describes an operation region of engine 200 with
reference to FIG. 6.

[0069] Referring to FIG. 6, operation region and operation point of engine
200 are represented by a combination of the rotational speed and the
torque. Engine 200 has an operational region falling within the following
range: engine torque Te<Tmax (maximum torque) and engine speed
Ne<Nmax (maximum rotational speed).

[0070] In a high output region, heat energy resulting from fuel combustion
in engine 200 is increased to cause increase of exhaust gas temperature.
Accordingly, catalyst 240 (FIG. 2) may have an excessively high
temperature. Hence, in such a high output region, the amount of fuel is
controlled to be increased to exceed the fuel injection amount that is in
accordance with the stoichiometric air-fuel ratio (i.e., OT amount
increase). The OT amount increase leads to enriching the fuel, thereby
decreasing the exhaust gas temperature. Accordingly, catalyst 240 is
protected from having an excessively high temperature.

[0071] As shown in FIG. 6, based on the characteristic of engine 200, a
boundary line 300 between a normal region 305 and an OT amount increase
region 310 can be determined in advance. In normal region 305, a fuel
injection amount based on the stoichiometric air-fuel ratio is applied.
In OT amount increase region 310, the OT amount increase is required.
Boundary line 300 is determined by specification of engine 200. Hence,
for any engine, boundary line 300 can be specified in advance when
designing the engine. Normal region 305 corresponds to a "first operation
region", and OT amount increase region 310 corresponds to a "second
operation region".

[0072] Also in hybrid vehicle 100 of the present embodiment, when the
operation point of engine 200 enters OT amount increase region 310, the
amount of fuel needs to be increased (OT amount increase). Such increase
in amount of fuel causes increase of unburned CO to result in
deteriorated emission, even when catalyst 240 is activated. Accordingly,
there arises a possibility of failing to satisfy the requirements of
severe regulations on emission.

[0073] To address this, the control device for hybrid vehicle 100 in the
present embodiment performs traveling control for preventing emission
from becoming deteriorated, as described below.

[0074]FIG. 7 is a flowchart illustrating the traveling control for the
hybrid vehicle in the first embodiment of the present invention. The
control process according to the flowchart shown in FIG. 7 is performed
repeatedly by PM-ECU 170 in a predetermined control cycle. It should be
noted that a process in each of steps in each of flowcharts such as the
one in FIG. 7 can be performed by software processing performed by the
ECU and/or hardware processing.

[0075] Referring to FIG. 7, in a step S100, PM-ECU 170 calculates total
driving power required by hybrid vehicle 100, in accordance with a
vehicle state (vehicle speed, pedal operation, or the like).
Representatively, the total driving power is calculated in accordance
with the accelerator position and the vehicle speed.

[0076] Then, in a step S110, PM-ECU 170 determines engine output power Pe
based on the total driving power calculated by step S100. Then, PM-ECU
170 determines an operation point (hereinafter, also simply referred to
as "engine operation point") of engine 200 for outputting engine output
power Pe.

[0077] In doing so, charging power Pchg required to charge battery 150 is
incorporated in engine output power Pe. For example, charging power Pchg
is determined based on a comparison between SOC at present and a SOC
control target (for example, 50% to 60%). Specifically, when the SOC is
lower than the control target and charging is required, Pchg>0 is set.
On the other hand, when the SOC is higher than the control target and
discharging is required, Pchg<0 is set.

[0078] Referring to FIG. 6 again, an operation line 315 for efficiently
operating engine 200 is set in advance. Operation line 315 is
representatively a set of operation points in which fuel consumption is
the best. Each of power contour lines 320 is a set of operation points in
which output powers are the same.

[0079] In step S110, a point of intersection between operation line 315
and power contour line 320 corresponding to engine output power Pe is
determined as the engine operation point.

[0080] Referring to FIG. 7 again, in a step S120, PM-ECU 170 determines
torque command values for first MG 110 and second MG 120 in accordance
with the engine operation point set in step S110, so as to generate the
total driving power. When the total driving power is insufficient with
the engine output attained with the determined engine operation point,
second MG 120 assists to compensate the insufficiency of torque.

[0081] As a result, in order to secure the total driving power, a ratio of
powers of first MG 110, second MG 120, and engine 200 in power
(hereinafter, also referred to as "total required power Ptl") required by
a whole of hybrid vehicle 100 are determined.

[0082] In a step S130, PM-ECU 170 determines whether or not the engine
operation point determined in step S110 is in OT amount increase region
310 shown in FIG. 6. When the engine operation point is in the OT amount
increase region (determined as YES in S130), PM-ECU 170 requests a change
of the ratio of powers in a step S140, Specifically, in step S140, the
engine output power is decreased and the output power of second MG 120 is
increased.

[0083] Further, PM-ECU 170 performs step S110 again to re-determine an
engine operation point that corresponds to the decreased engine output
power. In this way, the engine operation point is changed. PM-ECU 170
performs step S120 again to determine a ratio of powers in accordance
with the engine operation point thus changed.

[0084] Referring to FIG. 6 again, operation point P1 is in OT amount
increase region 310. Namely, when the engine operation point determined
in step S130 is P1, it is determined as YES in step S130. Further, when
the engine output power is decreased in step S140, the engine operation
point is changed from P1 toward P2.

[0085]FIG. 8 shows a nomographic chart in operation point P1 before the
change.

[0086] Referring to FIG. 8, the rotational speed and torque (Te) of engine
200 have values corresponding to operation point P1. The total driving
power acting on the driving shaft of hybrid vehicle 100 is a total of
engine direct-delivering torque Tep exerted from engine 200 and output
torque Tmg2 of second MG 120. Engine direct-delivering torque Tep is
given by Tep=-Tmg1/ρ, using output torque Tmg1 of first MG 110 and
gear ratio ρ of power split device 130. Usually, output torque Tmg1
of first MG 110 is positive (Tmg1>0) during starting of engine 200,
but is negative (Tmg1<0) during normal traveling.

[0087]FIG. 9 shows a nomographic chart in operation point P2 after the
change.

[0088] Now, FIG. 9 is compared with FIG. 8. In operation point P2, the
rotational speed and torque (Te) of engine 200 are decreased due to the
decrease of engine output power as compared with FIG. 8 (operation point
P1). Accordingly, engine direct-delivering torque Tep is decreased as
compared with that in operation point P2.

[0089] Meanwhile, in order to compensate the decrease of the engine
output, output torque Tmg2 of second MG 120 is increased as compared with
that in FIG. 8. As a result, the total of engine direct-delivering torque
Tep and output torque Tmg2 of second MG 120, which are the torque (i.e.,
vehicle driving power) acting on the driving shaft, is secured in an
equivalent manner to that in FIG. 8. In other words, even though the
engine operation point is changed from P2 to P1, the vehicle driving
power has a value corresponding to the total driving power calculated in
step S100.

[0090] Referring to FIG. 7 again, PM-ECU 170 performs step S130 again, and
determines whether or not the changed engine operation point is in OT
amount increase region 310. When the changed engine operation point is in
OT amount increase region 310, it is determined as YES in step S130. As a
result, the change of the ratio of powers in step S140 and the change of
the engine operation point in step S110 are performed again. Thus, the
processes of steps S110 to S140 are repeated until it is determined as NO
in step S130, i.e., until a ratio of powers is found such that the engine
operation point falls within normal region 305.

[0091] When the engine operation point is determined and falls within
normal region 305, PM-ECU 170 determines as NO in step S130 and proceeds
the process to a step S150. In step S150, PM-ECU 170 generates a control
target value for controlling engine 200 in accordance with the determined
engine operation point falling within normal region 305. The control
target value is sent to engine ECU 201.

[0092] As a result, according to the traveling control for the hybrid
vehicle in the first embodiment, engine 200 is operated in normal region
305 in which the fuel injection amount in accordance with the
stoichiometric air-fuel ratio is set, even in a vehicle state, such as a
high-speed traveling state or a vehicle accelerating state, in which
engine 200 would have been operated in the high output region (OT amount
increase region 310) in the conventional traveling control. Therefore, in
any vehicle state, engine 200 can be avoided from being operated in the
high output region (OT amount increase region 310) in which the amount of
fuel needs to be increased to prevent the catalyst from having an
excessively high temperature due to increase of exhaust gas temperature.
As a result, exhaust emission is prevented from being deteriorated.

[0093] Meanwhile, the output power decreased by changing the operation
point of engine 200 is covered by the increased output of second MG 120,
thereby securing the total driving power corresponding to the vehicle
state as determined in step S100. In this way, the exhaust emission of
the hybrid vehicle can be suppressed without causing insufficiency of the
vehicle driving power and the catalyst's excessively high temperature
resulting from increase of exhaust gas temperature.

Second Embodiment

[0094] In order to implement the traveling control for the hybrid vehicle
in the first embodiment, the torque compensation by increasing the output
of second MG 120 is required. Accordingly, when stored energy (SOC) of
battery 150 becomes insufficient, it becomes difficult to perform such
traveling control.

[0095] In the second embodiment, the following describes traveling control
including a process for securely implementing the torque compensation by
second MG 120. It should be noted that in the second and later
embodiments, the portions same as those in the first embodiment are not
particularly mentioned and described.

[0096] FIG. 10 is a flowchart illustrating the traveling control for the
hybrid vehicle in the second embodiment of the present invention. The
control process according to the flowchart shown in FIG. 10 is performed
repeatedly by PM-ECU 170 in a predetermined control cycle.

[0097] Comparing FIG. 10 with FIG. 7, the traveling control for the hybrid
vehicle in the second embodiment is different in that steps S170 to S190
are performed when it is determined as NO in step S130, i.e., after an
engine operation point falling within normal region 305 is determined.
Control processes in steps other than these are the same as those
described in FIG. 7 and therefore are not described repeatedly.

[0098] When the engine operation point falling within normal region 305 is
determined (determined as NO in S130), PM-ECU 170 compares the SOC at
present with a reference value Sth in step S170. Reference value Sth can
be set to provide a margin for a SOC region in which it is difficult to
increase the torque of second MG 120. For example, reference value Sth is
at a level lower than the normal SOC control target.

[0099] When the SOC at present is lower than reference value Sth
(determined as YES in S170), PM-ECU 170 changes the engine operation
point in step S180 so as to increase the engine speed with the output
power of engine 200 being kept constant. Further, in step S190, PM-ECU
170 increases an amount of electric power generated by first MG 110, by
utilizing the increased engine speed (step S180) resulting from the
change of operation point.

[0100] Referring to FIG. 11, operation point P2 is the engine operation
point set in normal region 305 by the traveling control in the first
embodiment. When the SOC is low, an operation point P3, in which engine
speed Ne is increased on power contour line 320, is set in step S180
(FIG. 10). In this way, the engine speed can be increased while
maintaining the ratio of powers with which the operation point of engine
200 falls within normal region 305.

[0101] FIG. 12 shows a nomographic chart in operation point P2. FIG. 13
shows a nomographic chart in operation point P3. The nomographic chart of
FIG. 12 is equivalent to the nomographic chart shown in FIG. 9.

[0102] Comparing FIG. 13 with FIG. 12, in operation point P3, engine
torque Te is decreased whereas the engine speed is increased. As a
result, the rotational speed of first MG 110 is increased to attain the
same rotational speed of the driving shaft (i.e., vehicle speed). The
electric power generated by first MG 110 is in proportion to a product of
the torque and the rotational speed. Hence, by changing the engine
operation point from P2 to P3, the electric power generated by first MG
110 can be increased.

[0103] Referring to FIG. 10 again, when the SOC at present is higher than
reference value Sth (determined as NO in S170), PM-ECU 170 skips the
processes of steps S180, S190. Alternatively, in the case where the
engine operation point has not been changed in steps S130, S140, PM-ECU
170 may skip the processes of steps S180, S190 irrespective of the SOC.

[0104] In step S150, PM-ECU 170 generates a control target value for
controlling engine 200, in accordance with the engine operation point
determined in step S120 or the engine operation point changed in S180.
Then, PM-ECU 170 sends it to engine ECU 201. In each of the cases, the
engine operation point is determined to fall within normal region 305 as
described above.

[0105] Thus, according to the traveling control for the hybrid vehicle in
the second embodiment, electric power required to increase the output of
second MG 120 by the traveling control in the first embodiment can be
generated by increasing the amount of electric power generated by first
MG 110, even when the stored energy of battery 150 is little (the level
of the SOC is low).

[0106] Alternatively, as described above, by performing the processes of
steps S170 to S190 only when the engine operation point is changed (S130,
S140), the change of the engine operation point for increasing the
electric power generated by first MG 110 can be necessary minimum. In
other words, when the SOC is low (SOC<Sth) even though the change of
the engine operation point is necessary, electric power for increasing
the output of second MG 120 is generated. On the other hand, when this is
not necessary, engine efficiency can be prevented from being decreased.

[0107] Further, by configuring to always perform the processes of steps
S170 to S190, the SOC of the battery can be made higher than reference
value Sth, thus preparing for the increase of the output of second MG 120
in the traveling control in the first embodiment even in a vehicle state
in which the output of second MG 120 is not high.

Third Embodiment

[0108] A ratio of powers with which the engine operation point falls
within OT amount increase region 310 (FIG. 6) is required only in a
certain special vehicle state (high-speed/high-load state) in which the
accelerator position becomes large during high vehicle speed. For
example, the engine operation point is set to fall within the OT amount
increase region, in a traveling state involving accelerating for
overtaking or traveling up a hill during high-speed traveling.

[0109] Thus, in the third embodiment, the following describes traveling
control for increasing the level of the stored energy (SOC) of battery
150 in advance in the case where there is a possibility that such a
special vehicle state takes place.

[0110] FIG. 14 is a flowchart illustrating the traveling control for the
hybrid vehicle in the third embodiment of the present invention. The
control process according to the flowchart shown in FIG. 14 is performed
repeatedly by PM-ECU 170 in a predetermined control cycle.

[0111] Comparing FIG. 14 with FIG. 7, the traveling control for the hybrid
vehicle in the third embodiment is different in that steps S200 to S220
are further performed when it is determined as NO in step S130, i.e.,
after the engine operation point falling within normal region 305 is
determined. Control processes in steps other than these are the same as
those described in FIG. 7 and therefore are not described repeatedly.

[0112] In step S200, PM-ECU 170 determines whether or not the engine
operation point has been changed in steps S130, S140. When the engine
operation point has not been changed (determined as NO in S200), PM-ECU
170 determines in step S210 whether or not the vehicle is in a traveling
state that requires high SOC control (charge level increasing control)
for preliminarily increasing the charge level (SOC) of battery 150, so as
to prepare for the increase of the output of second MG 120 in the
traveling control according to the first embodiment. Namely, in step
S210, it is determined whether or not the traveling state satisfies a
predetermined condition for the high SOC control.

[0113] The condition for the high SOC control is set to cover a traveling
state expected to involve occurrence of a vehicle state in which engine
200 is requested to achieve a high output such that the engine operation
point is changed by the traveling control according to the first
embodiment. For example, when high-speed traveling equal to or faster
than a predetermined speed such as 100 km/h continues for a predetermined
period of time, the condition for the high SOC control is established,
whereby it is determined as YES in step S210. Alternatively, also when it
is predicted based on navigation information or the like that the vehicle
will travel up a hill at a high vehicle speed, the condition for the high
SOC control is established based on slope and distance of the hill or a
traveling distance to the hill.

[0114] When the condition for the high SOC control is established
(determined as YES in S210), PM-ECU 170 turns on the high SOC control in
step S220 so as to increase the SOC level. As the high SOC control, the
SOC control target of battery 150 is increased to exceed the normal SOC
control target (such as 50% to 60%), thereby facilitating charging on
battery 150. In this way, charging power Pchg for charging battery 150 to
achieve the temporarily increased SOC control target is incorporated in
total required power Ptl, thereby forcibly charging battery 150.
Alternatively, the high SOC control can be implemented by offsetting
charging power Pchg in the positive direction, which is determined based
on the SOC at present and the SOC control target. While the high SOC
control is on, the ratio of powers of engine 200, first MG 110, and
second MG 120 and the operation point are determined based on charging
power Pchg thus increased, thereby facilitating charging on battery 150.

[0115] When the condition for the high SOC control is not established
(determined as NO in S210), PM-ECU 170 skips the process of step S220.
Further, when the engine operation point has been changed (determined as
YES in S200), the processes of steps S210, S220 are skipped. This is
because the high SOC control in step S220 is to prepare for the increase
of power consumption of second MG 120 due to the change of the engine
operation point. When step S220 is not performed, the high SOC control is
automatically turned off.

[0116] In step S150 similar to that in FIG. 7, PM-ECU 170 generates a
control target value for controlling engine 200, in accordance with the
determined engine operation point falling within normal region 305. It
should be noted that when the high SOC control is turned on, the high SOC
control is reflected, from at least next control cycle, in determining
the ratio of powers and the operation point, but the ratio of powers and
the operation point may be modified in this control cycle.

[0117] FIG. 15 shows exemplary traveling control for the hybrid vehicle in
the third embodiment.

[0118] Referring to FIG. 15, when high-speed traveling exceeding a
predetermined speed Vt continues for a predetermined time or more during
a period of time t1 to time t2, it is determined as YES at time t2 in
step S200 of FIG. 14.

[0119] As a result, from time t2, the high SOC control is performed to
increase the SOC of battery 150. Then, in acceleration traveling during a
period of time t3 to time t4, the traveling control described in the
first embodiment is performed. Namely, the output of second MG 120 is
increased in order to bring the engine operation point out of OT amount
increase region 310 (FIG. 6). Although electric power needs to be
supplied from battery 150 so as to increase the output of second MG 120,
this electric power can be surely secured by the SOC increased by the
high SOC control performed during the period of time t2 to time t3.

[0120] Thus, according to the traveling control for the hybrid vehicle in
the third embodiment, the SOC of battery 150 can be increased in advance
through the high SOC control in the case where there is a possibility
that the vehicle will be brought into a vehicle state in which engine 200
is requested to attain a high output requiring a change of the engine
operation point. As a result, the traveling control according to the
first embodiment, which involves the increase of the output of second MG
120, can be performed more securely.

[0121] It should be noted that the traveling control according to the
second embodiment can be combined with the traveling control according to
the third embodiment. For example, in the case where the engine operation
point has been changed in steps S130, S140 (in the case where it is
determined as YES in S200), the control process may be adapted to perform
the processes of steps S170 to S190 of FIG. 7. In this way, even in the
case where the SOC of battery 150 is decreased because the
high-speed/high-load vehicle state continues for a long period of time,
electric power required for increasing the output of second MG 120 can be
generated by increasing the amount of electric power generated by first
MG 110.

Fourth Embodiment

[0122] Depending on a vehicle state, the hybrid vehicle travels only using
the output of second MG 120 with engine 200 being stopped, as described
above. Namely, in the hybrid vehicle, engine 200 is intermittently
operated in accordance with a vehicle state.

[0123] Hence, in the case where a total amount of exhaust emission in a
certain traveling mode is regulated, it is also important to suppress
exhaust emission when starting engine 200 due to the intermittent
operation of the engine.

[0124] In the fourth embodiment, the following describes engine starting
control for suppressing exhaust emission, as one exemplary traveling
control for the hybrid vehicle.

[0125] FIG. 16 is a waveform diagram illustrating normal engine starting
control for the hybrid vehicle. Each of FIG. 16 and FIG. 17 shows
waveforms for operations in starting the engine when starting the
vehicle.

[0126] Referring to FIG. 16, hybrid vehicle 100 increases the engine speed
by motoring engine 200 using positive torque of first MG 110, and starts
fuel injection. When starting the engine, the fuel injection amount is
temporarily increased in order to secure the engine output. In other
words, by temporarily rendering the air-fuel ratio richer than the
stoichiometric air-fuel ratio, energy for surely starting engine 200 is
secured. However, this increased amount of fuel causes generation of
unburned CO, thus resulting in deteriorated exhaust emission.

[0127] First MG 110 generates positive torque for motoring, and thereafter
the output torque is reduced. At the time of completion in starting the
engine, first MG 110 generates negative torque. In other words, first MG
110 is adapted to generate negative torque before the engine speed
reaches target rotational speed N0 during the engine start.

[0128] As a result, the rotational speed of first MG 110 is monotonously
increased toward a steady-state rotational speed N1, which is attained at
the time of completion of the engine start. This steady-state rotational
speed N1 is determined by target rotational speed N0 of engine 200 at the
start of the engine and the gear ratio (p) of power split device 130.

[0129] Thus, in the normal engine start of hybrid vehicle 100, the
motoring by first MG 110 is auxiliary and surely secures energy for
starting the engine by means of the increase in amount of fuel. It is
understood that until completion of the starting of engine 200, driving
power for attaining vehicle speed is covered by the torque of second MG
120.

[0130] FIG. 17 shows a waveform diagram illustrating engine starting
control according to traveling control for the hybrid vehicle in the
fourth embodiment. In FIG. 17, for comparison, the waveforms for
operations in the normal engine starting control shown in FIG. 16 are
indicated by dotted lines.

[0131] Referring to FIG. 17, in the engine starting control according to,
the fourth embodiment, the motoring of first MG 110 is performed for a
longer period of time than that in the normal control (FIG. 16).
Particularly, first MG 110 generates positive torque until the engine
speed reaches target rotational speed N0 upon the starting thereof.
Accordingly, unlike in FIG. 16, engine 200 can be started without
increasing the amount of fuel. The rotational speed of first MG 110 is
also raised quickly as compared with the case of FIG. 16. Particularly,
this engine starting control is different from FIG. 16 in that the
rotational speed of first MG 110 is increased to exceed steady-state
rotational speed N1 before the completion of engine start and then
becomes steady-state rotational speed N1 upon the completion of engine
start.

[0132] Further, in order to cover the slower increase of the engine speed
than that in FIG. 16, the output torque and rotational speed of second MG
120 are increased more quickly than those in FIG. 16. In this way, change
in vehicle speed during the engine start is substantially the same as
that in FIG. 16.

[0133] Thus, in the traveling control for the hybrid vehicle in the fourth
embodiment, the motoring by first MG 110 is enhanced, thus setting the
fuel injection amount in accordance with the stoichiometric air-fuel
ratio even when starting the engine. As a result, unlike the normal
engine starting control involving the increase of fuel amount as shown in
FIG. 16, exhaust emission is never deteriorated upon the engine start. As
a result, a total of amounts of emissions during traveling involving
engine start and engine stop can be suppressed. The traveling control
according to the fourth embodiment can be appropriately combined with the
traveling control described in the first to third embodiments.

[0134] Thus, according to the traveling controls for the hybrid vehicle in
the first to fourth embodiments, the amount of fuel does not need to be
increased to prevent the catalyst from having an excessively high
temperature and also the amount of fuel does not need to be increased
upon the engine start, thereby suppressing the exhaust emission
throughout the period of traveling. Accordingly, it is expected to
satisfy the requirements of severe regulations on emission.

[0135] The traveling control for the hybrid vehicle in the present
embodiment can be also applied to configurations different from hybrid
vehicle 100 illustrated in FIG. 1 in terms of driving systems of hybrid
vehicles. Specifically, the traveling controls according to the first to
third embodiments can be applied to any configuration in which both the
engine and the motor can generate vehicle driving power such as a
parallel type hybrid vehicle, irrespective of the number of motors (motor
generators) and the configuration of the driving system provided.
Further, the traveling control according to the fourth embodiment can be
also applied to any configuration provided with a motor that motors the
engine when starting the engine.

[0136] The embodiments disclosed herein are illustrative and
non-restrictive in any respect. The scope of the present invention is
defined by the terms of the claims, rather than the embodiments described
above, and is intended to include any modifications within the scope and
meaning equivalent to the terms of the claims.

INDUSTRIAL APPLICABILITY

[0137] The present invention can be applied to a hybrid vehicle including
an engine and a motor as a driving power source.